Urban storm-runoff modelling; Madison, Wisconsin

Open-File Report 79-1273

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Abstract

Four urban basins in Madison, Wisconsin, have been modeled to determine the effects that (1) physical changes to storm-sewer conduits, and (2) increased runoff detention and infiltration would have on peak discharge and runoff volume attenuation. Various storm-water-management options were simulated using the Illinois Urban Drainage Area Simulator (ILLUDAS). A brief evaluation was made of a modified version of ILLUDAS that simulates quality of urban runoff. Some notable simulation results were that a 25 percent storm-sewer slope reduction yielded only a 3 percent peak-discharge reduction, and increasing storm-sewer roughness by increasing Manning 's n from 0.013 to 0.040 decreased peak discharge about 10 to 20 percent. Detention of 10 percent of runoff throughout each basin also reduced peak discharge about 10 to 20 percent. Infiltration of all parking-lot runoff reduced peak discharges 5 to 24 percent. Peak discharges were reducted 71 to 88 percent by substituting porous pavement for conventional pavement. Draining 90 percent of the residential rooftops onto lawns instead of driveways reduced peak discharge from 7 to 31 percent. Runoff-volume reduction was similarly reduced for the induced infiltration simulations. Storage requirements for hypothetical storm-water-treatment plants ranged from 2.6 to 29 acre-feet for the smallest and largest basins, respectively, with a treatment capacity of 25 cubic feet per second. A brief evaluation to simulate storm-runoff quality resulted in the model computing loads 7 to 11 times greater than observed loads for ammonia nitrogen and orthophosphate. The ratio of simulated to observed loads for nitrates, organic nitrogen, total phosphate, and total solids ranged from 0.50 to 1.8. Observed loads are doubtful because of the sparsity of water-quality data. (Kosco-USGS)